120 tweets per minute
TwittStorm is a real-time tweet visualizing website. You setup your TwittStorm for a certain keyword and get a personalized page in a cool design which constantly loads in all related tweets coming in as you watch the page. Test it here: www.twittstorm.com. It is free, you do not have to register for it and works with any keyword. A tweet generating 12 tweets a minute is called a Breeze, with up to 30 tweets per minute it is a Gale and everything with 120 tweets per minute or more is called a TwittStorm. The start page of www.twittstorm.com informs you about the current TwittStorms going on.
TwittStorm makes use of the Twitter Streaming API, allowing to receive real time tweets from around the world. Have fun!